For ion trap type mass spectrometers, the pumping capability is not efficiently used with a traditional constantly open API. The ions are usually allowed to pass into the ion trap for only part of each scan cycle but neutrals are constantly leaked into the vacuum manifold and need to be pumped away to keep the pressure at the low levels typically needed for mass analysis. Although the mass analysis using an ion trap usually requires an optimal pressure at several milli-torr or less, ions can be trapped at a much higher pressure. (Shaffer, S. A.; Tang, K. Q.; Anderson, G. A.; Prior, D. C.; Udseth, H. R.; Smith, R. D. Rapid Communications in Mass Spectrometry 1997, 11, 1813-1817).
Taking advantage of this characteristic of an ion trap, an alternative atmospheric pressure interface, a discontinuous atmospheric pressure interface (DAPI), has been developed to allow maximum ion transfer at a given pumping capacity for mass spectrometers containing an ion trapping component (Ouyang et al., U.S. patent application Ser. No. 12/622,776 and PCT application number PCT/US2008/065245). The concept of DAPI is to open its channel during ion introduction and then close it for subsequent mass analysis during each scan. An ion transfer channel with a much bigger flow conductance can be allowed for a DAPI than for a traditional continuous API. The pressure inside the manifold temporarily increases significantly when the channel is opened for maximum ion introduction. All high voltages can be shut off and only low voltage RF is on for trapping of the ions during this period. After the ion introduction, the channel is closed and the pressure can decrease over a period of time to reach the optimal pressure for further ion manipulation or mass analysis when the high voltages can be is turned on and the RF can be scanned to high voltage for mass analysis.
A discontinuous API opens and shuts down the airflow in a controlled fashion. The pressure inside the vacuum manifold increases when the API opens and decreases when it closes. The combination of a discontinuous atmospheric pressure interface with a trapping device, which can be a mass analyzer or an intermediate stage storage device, allows maximum introduction of an ion package into a system with a given pumping capacity.